A clear understanding of the nature of quantitative genetic characters requires knowing how many genes and what types of developmental influences are included in the general category "polygenic variation". Almost any character can respond to artificial selection, but very little is known about the ways in which the polygenes, or genes of small effect, modify the development of these traits. Using Drosophila melanogaster as a model organism, breeding programs have been developed to permit isolation of individual polygenic loci accounting for the majority of the genetic difference between strains differing in the expression of a quantitative character (Thoday, 1961, Nature 191: 368-370; Thompson and Thoday, 1974, Heredity 33: 430-437). Pilot studies conducted at the University of Cambridge and later at the University of Oklahoma have helped identify a variety of morphological and behavioral traits in which studies of gene number and development can be carried out successfully. This study has two primary phases. First, whole chromosome substitutions and polygene location assays will be carried out to estimate the number of genes accounting for the majority of the genetic variance in several model quantitative traits. Second, selected polygenic loci, isolated in the first, or location phase, will be submitted to a series of studies to clarify the contribution each locus makes to the observed modification or variation in development. These assays will include histological descriptions of the developing organ in the presence and absence of the isolated polygenic locus. Where feasible, electrophoretic and immunological comparisons of proteins will be made. The information gathered in these studies will be synthesized into a review of the genetics of quantitative traits and will, hopefully, serve as a better foundation for understanding and modifying economically important quantitative genetic characters.